This invention relates to a transfer switch and, more particularly, to a separately-derived transfer switch having a lockout sequencing arrangement that sequences manual switching of a load between power supplies to prevent open neutral transients during the switching.
In an electrical supply system, there are occasions when an alternate source of electric power is necessary or desirable. For example, the capability of switching from utility power to emergency generator power is important for businesses, hospitals and industries, and is also employed in residential applications.
It is desirable for separate electrical circuits, or separate groups of electrical circuits, to be arranged so that when one group of circuits is switched to a conductive state, another group of circuits is switched to a non-conductive state so as to prevent power supply to the circuits from two different power sources at the same time, e.g. from both a utility power supply and a generator power supply. In an arrangement such as this, a switch is typically provided for each power source to control the supply of electrical power. Accordingly, it is important to ensure that the switches are prevented from both being in the ON position at the same time, to ensure that power is supplied to the switch from only one power source.
To this end, switch interlocks have been developed that are designed to prevent simultaneous connection of circuits to two different power sources, such as described in U.S. Pat. No. 6,096,986, the disclosure of which is incorporated herein and assigned to the assignee of the present application. For some transfer switches, providing linkages that prevent the inadvertent switching of circuits to two power supplies is sufficient. However, for some types of transfer switches, more than an interlock is needed. For instance, if a separately-derived transfer switch is not properly switched, open neutral switching transients may be introduced.
The present invention is directed to sequencing lockout arrangement for use with a separately-derived transfer switch that sequences manual switching of main and generator side switches to prevent the introduction of open neutral switching transients. A separately-derived transfer switch typically includes a utility mains breaker and a utility mains neutral switch as well as a generator mains breaker and a generator mains neutral switch. In one embodiment of the present invention, two slidable lockout sequencers together with a rocker lockout functions to sequence switching of a load from one power source to another power source. In this embodiment, seven separate operations must be performed to switch the load between power sources. In another embodiment, the utility mains neutral and generator mains neutral switches are linked together such that switching of the utility mains neutral to a conductive position automatically switches the generator mains neutral switch to a non-conductive position, and vice-versa. In this embodiment, five separate operations are required to switch a load between power sources.
The slidable lockout sequencers together with the rocker lockout in the first-mentioned embodiment allow only one of the utility mains breaker, the utility mains neutral switch, the generator mains breaker, and the generator mains neutral switch to be switched at a time. Moreover, the lockout sequencers and the rocker lockout cooperate such that a pre-defined order or sequence of the one-at-a-time switching must be followed to switch a load from one power source to another. The slidable lockout sequencers similarly define the sequence of switching with the interlinked neutral switches of the second-mentioned embodiment. Thus, in both embodiments, the slidable lockout sequencers provide limited and ordered switching of the utility and generator switches.
Thus, it is one object of the present invention to provide a lockout arrangement for use with a separately-derived transfer switch that is operable to prevent open neutral switching transients.
It is another object of the present invention to provide a separately-derived transfer switch having a pair of slidable members that restrict movement of switch handles such that a load is switched from one power source to another in a pre-defined, unalterable sequence.
In accordance with one aspect of the present invention, these and other objects are achieved with a lockout arrangement having a first slidable lockout movable between a first position and a second position, and a second slidable lockout movable between a third position and a fourth position. A third lockout is movable between a fifth and a sixth position. Further, the third lockout is only movable from the fifth position to the sixth position if the first slidable lockout is in the second position.
In accordance with another aspect, the invention is directed to a separately-derived transfer switch having a first mains breaker associated with a first power supply and a second mains breaker associated with a second power supply. The transfer switch further includes a first mains neutral switch and a second mains neutral switch associated with the first and the second power supplies, respectively. A lockout sequencing arrangement has a first lockout that restricts simultaneous switching of the first mains breaker and the first mains neutral switch and further includes a second lockout that restricts simultaneous switching of the second mains breaker and the second mains neutral switch. In a further aspect, the lockout sequencing arrangement includes a third lockout that prevents the first mains neutral switch and the second mains neutral switch from being both in a conductive position simultaneously. In one embodiment, the third lockout interlinks the first and the second neutral switches.
The present invention may also be embodied in a method of disconnecting a load from a utility power supply and connecting the load to a generator. The method includes switching a first mains breaker from an ON position to an OFF position and then sliding a first side lockout from a first position to a second position. The method continues with switching a first mains neutral switch from an ON position to an OFF position followed by switching a second mains neutral switch from an OFF position to an ON position. A second side lockout may then be switched from a third position to a fourth position followed by switching a second main breaker from an OFF position to an ON position. The above series of steps may be performed in a reverse order to disconnect the load from the generator and to connect the load to the utility power supply. In one embodiment, the first mains neutral and the second mains neutral switching operations are performed simultaneously by interlinking the neutral switches.
Various other features, objects and advantages of the invention will be made apparent from the following description taken together with the drawings.